Bumpless transfer circuits for process control systems

ABSTRACT

A process control system including automatic and manual control subsystems which alternatively regulate current supplied to a control element. The system includes means for providing bumpless transfer from one subsystem to the other. Memory capacitors in feedback paths to the subsystems store charges proportional to the voltage across the control element. The stored charges provide feedback bias to each amplifier in the subsystem to which control is being transferred.

United States Patent [151 3,662,276 Hyer 1 May 9, 1972 [54] BUMPLESSTRANSFER CIRCUITS FOR Primary Eramt'ncr-Roy Lake AssistantExaminer-James B. Mullins Attorney-William S. Wolfe, Frank L. Neuhauser,Oscar B. Waddell and Joseph B. Forman ABSTRACT A process control systemincluding automatic and manual control subsystems which alternativelyregulate current supplied to a control element. The system includesmeans for providing bumpless transfer from one subsystem to the other.Memory capacitors in feedback paths to the subsystems store chargesproportional to the voltage across the control element. The storedcharges provide feedback bias to each amplifier in the subsystem towhich control is being transferred,

7 Claims, 2 Drawing Figures PROCESS CONTROL SYSTEMS [72] Inventor:Donald Roy Hyer, Lynnfneld, Mass.

[73] Assignee: General Electric Company [22] Filed: Oct. 2, I970 [2]]Appl. No.: 77,563

[52] U.S. Cl. .3301] A, 3 l8/59l, 328/l 328/71, 330/51. 330/85 s l] Int.Cl. ..|-|03r 1/32 [58] Fieldofselrch ..330/$l, l A,85;328/l,7l; 3l8/59l,6lS, 68l

[56] Relerences Cited UNITED STATES PATENTS 3,422,457 1/1969 Koppel..3l8/59l {T lImTafccTmTaoI 5165155. 'M1/65 166 l v r' 1 -'W\r- :-1- l64 I L l- J T- ESEHZRZE FnEssTmE CONTROLLER SP I as PATENTEDMAY 9 I972FIGI CONTROL ELEMENT MODE CONTROL OUTPUT SELECTOR I CIRCUITRY DISCHARGEPRESSURE TRANSDUCER CURRENT TRANSDUCER SUCTION PRESSURE TRANSDUCER R M am EE Lmw k L ML L ARS RUL l- OUL UTVI Aso R0 0 NNS Han RR TSR A B C T ETM U S N RN S D w w PM E E L E 3 a 2 2 CONTROL ELEMENT INVENTOR DONALD R.HYER,

BY @WYL \Q Q-QRJ;

HIS ATTORNEY.

CURRENT CONTROLLER sucnow PRESSURE CONTROLLER Pv BACKGROUND OF THEINVENTION The present invention pertains to process control systems andmore particularly to means for providing bumpless transfer betweenautomatic and manual modes of process control.

In a closed loop process control system, a number of process variablesare monitored by transducers each of which generates an electricalsignal having a magnitude representative of the existing value of aparticular process variable. Each transducer signal is applied to aprocess controller along with a setpoint signal representing either anoptimum or a limiting value for the process variable, depending on thespecific application. For the purposes of description, it should beassumed that the setpoint signal represents a limiting value instead ofan optimum value. If a comparison between the transducer signal,hereatter referred to as the process variable signal, and the setpointsignal shows the process variable is exceeding the setpoint, thecontroller transmits a control signal to a control element for theprocess variable. The control element alters the operating condition ofa variable-controlling device such as a valve to drive the variablewithin the permissible range of values bounded by the setpoint. Controlof a process variable through a closed loop including a transducer, aprocess controller and a control element is referred to as automaticcontrol.

For some applications, it may be desirable to supplement automaticcontrol circuits with manual control circuits in which an adjustablevoltage source provides a controlling current for the control element.Manual control circuits are made part of normally automatically operatedsystems for several reasons. Manual control circuits assure that afailure in the transducer or process controller sections of a closedloop will not result in major disruptions of the process beingcontrolled. Also, manual control circuits permit an experienced operatorto control the process during startup, while "tuning" the process orwhile temporarily changing the level of operation of the process.

Where a process control system includes both an automatic controlsubsystem and a manual control subsystem, problems can arise whencontrol of a process variable is transferred from the controllingsubsystem to the idle or non'controlling subsystem. Unless the output ofthe idle subsystem and the controlling subsystem are approximately thesame during the transfer, a sudden current transition or current bumpwill occur in the current supplied to the control element. This bump orrapid change may temporarily disrupt the process with consequent loss ofproduct quality or production time.

Attempts have been made to provide bumpless transfer by including amanual balance position between automatic and manual positions of a modecontrol switch. With the switch in the manual balance position, theoutput of the manual control subsystem is manually adjusted to equal theexisting output of the automatic control subsystem. Control istransferred only after the outputs are equalized. Other attempts havebeen made to accomplish bumpless transfer by including amplifiers whichare continually controlled by electromechanical servo systems to produceoutputs which track the output of the circu-it in control. The manualbalance procedure is not fully satisfactory since it is time-consumingand only as accurate as the operator performing the balance procedure.The continuous servo system circuits are not fully satisfactory eithersince they tend to be bulky and costly.

Moreover, in some systems, a plurality of controllers, each responsiveto a different process variable, are used to control a single controlelement which influences each of the process variables. The controllerinputs are scaled so that the most extreme controller output actuallydrives the control element during the automatic mode of operation. Sincesystems such as this include a plurality of amplifiers, any one of whichmay provide a controlling current at any time depending on the conditionof its input, the disadvantages of the manual balancing procedures or ofthe servo system balancing arrangement are greatly multiplied.

SUMMARY OF THE INVENTION The present invention overcomes these and otherdissdvantages of the prior art systems. A system incorporating thepresent invention is capable of transferring between automatic andmanual modes of operation without generating any significant currentbumps during transfer. The system includes an automatic controlsubsystem with one or more process controllers, each of which has inputsrepresenting a particular process variable and a particular setpoint forthat variable, an output, and a negative feedback path including aseries-connected capacitor. The system also includes a manual controlsubsystem including an adjustable voltage source which drives a manualcontrol amplifier. During automatic operation switching means connectsone of the process controllers to a control element. During manualoperation, the switching means connects the manual control amplifier tothe control element. A system feedback path includes a first branchwhich connects the control element to the feedback paths of the one ormore controllers and a second branch which connects the element to themanual control amplifier. The second branch includes a series-connectedcapacitor. When the system is operating in the automatic mode, thecapacitor in the second branch is charged to the voltage appearingacross the control element. When the system is operating in the manualmode, each of the capacitors in the feedback paths of the one or morecontrollers is charged to the voltage across the control element. Upontransfer of control from one subsystem to the other, the chargedcapacitors assure that the output of the selected subsystem isapproximately the same as the voltage appearing across the controlelement just prior to the transfer.

DESCRIPTION OF THE DRAWINGS While the specification concludes withclaims particularly pointing out and distinctly claiming that which isregarded as the present invention, details of a preferred embodiment ofthe invention along with further objects and advantages may be morereadily ascertained from the following detailed description when read inconjunction with the accompanying drawings in which:

FIG. I is a block diagram of a closed loop process control system intowhich the present invention may be incorporated; and

FIG. 2 is a schematic diagram of certain portions of the closed loopprocess control system, showing the present invention in detail.

DETAILED DESCRIPTION System Block Diagram FIG. 1 shows a typicalapplication for a process control system including a multiple-controllerautomatic control subsystem and a manual control subsystem foralternatively regulating the current supplied to a single controlelement. In that figure, the device being controlled is a valve 10located on the discharge side of a booster pump 12 in a fluid pipeline14. The downstream or discharge pressure in the pipeline 14 is monitoredby a discharge pressure transducer [6 whereas the upstream or auctionpressure is monitored by a similarly constructed suction pressuretransducer 18. The load current in the pump motor is monitored by acurrent transducer 20. The transducers l6, l8 and 20 generate electricalsignals which are applied to corresponding process controllers 22, 24and 26. Under normal operating conditions, the output of the dischargepressure controller 22 is passed through output selector circuitry 28 toa mode control or switching means 30 which determines whether a controlelement 32 is to respond to the output of one of the controllers 22, 24and 26 or to the output of a manual control subsystem 34 also connectedto the mode control 30.

To illustrate a typical application for a multiple-controller closedloop process control system, it is assumed that the control element 32is under the control of an automatic control subsystem consisting of:transducers 18, 20, 22; controllers 22, 24, 26; and output selectorcircuitry 28. As was indicated before, during normal operation thedischarge pressure in the pipeline 14 is monitored to control theposition of the valve 10. if the pipeline 14 becomes blocked upstream ofpump 10, an abnormal drop in the suction pressure causes a similar dropin the discharge pressure. The discharge-pressure controller, if actingalone, would attempt to remedy the pressure drop by opening the valve tobring the discharge pressure up to a setpoint. Opening the valve 10would result in a further drop in suction pressure. Eventually, thesuction pressure would reach subnormal levels at which booster pump 12would vapor-lock and burn out. Another undesirable condition may occurin the pipeline where the suction pressure is high but the dischargepressure is low. This combination of pressures leads to high motorcurrents which can eventually destroy the pump motor.

To safely control the operation of the pipeline, a selector controlsystem is used in which discharge pressure and motor currents are heldto high limits and suction pressure is held to a low limit. Normally, adirect-acting valve is used in this type of application so that it willclose on loss of signal. Consequently, reverse-acting controllers arerequired in the discharge pressure and motor current controller loops sothat valve 10 will tend to close if the discharge pressure or the motorcurrent exceeds a predetermined limit. With suction pressure being heldto a low limit, the control action must be such as to close the valve ifthe suction pressure falls below that limit. This requires the use of adirect-acting controller.

ln the system as described, the output selector circuitry passes thelowest of the controller signals which allows the discharge-pressurecontroller 22 to operate the valve 10 during normal operatingconditions. However, if the suction pressure drops below a limit, itssignal becomes the lowest and the controller 24 assumes control of thevalve 10 through control element 32. Similarly, if the motor currentexceeds its setpoint, the current controller 26 will have the lowestoutput signal of the three controllers and will act to control the valve10 through the control element 32.

For a number of reasons, an operator may wish to remove the control ofthe valve 10 from the closed loop and assume control of the valveposition himself. To do this, he uses mode control 30 to disconnect theoutput selector circuitry 28 from the control element 32 whileconnecting the manual control subsystem 34 to control element 32. Byadjusting the current provided by the manual control subsystem 34, theoperator controls the current provided to control element 32 and thusthe position of the valve 10.

During the transfer of control from the automatic control subsystem tothe manual control subsystem (or from manual to automatic) thedifi'erent levels of outputs of the two subsystems can cause bumps inthe current supplied to the control element 32. These bumps may causethe valve 10 to hunt" for a new position, disturbing the fluid flow inthe pipeline 14 or the process to which the pipeline is connected. Thepurpose of the present invention is to eliminate the current bumpsduring the transfer of control from one subsystem to the other.

Circuit Description Referring to FIG. 2, there are more detailedschematic diagrams of certain of the elements shown only in blockdiagram form in FIG. I. At the same time, the elements in the closedloop system which are not essential to an explanation of the presentinvention have been omitted from FIG. 2. The discharge pressurecontroller 22 is shown in greater detail whereas the current controller26 and the suction pressure controller 24 are shown only in blockdiagram form. The controllers 24 and 26 are similar to the controller 22except that the suction pressure controller 24 is direct-acting ratherthan reverse-acting like controllers 26 and 22. The changes incontroller connections which are needed to make a controllerdirect-acting rather than reverse-acting are known to those skilled inthe art and it is deemed unnecessary to illustrate those connections. Inthe following discussion, reference is made to components in controller22. Since controllers 22, 24, 26 are connected in parallel, it should beunderstood that any such reference applies to equivalent components ineach of the other controllers.

In the discharge pressure controller 22, a signal representative of thedischarge pressure is applied at one input PV to a high impedanceamplifier 36. A second electrical signal representing a setpoint or highlimit for the discharge pressure is applied to an input terminal SP of asecond high input impedance amplifier 38. The deviation between theprocess variable and the setpoint is determined by combining the outputsof the amplifiers 36 and 38 in a differential error amplifier 40. Theoutput of amplifier 40 is applied to a circuit 42 consisting of theparallel combination of resistor 44 and capacitor 46. The output of thecircuit 42 is applied to a first input 48 of controller amplifier 50. Asecond input 52 to the controller amplifier 50 is connected to a COMMONterminal 54 and to the output of circuit 42 through a relay-controlledswitch 90. The triangular symbol representing the COMMON terminal isused throughout FIG. 2 to indicate that each of the designated points istied electrically to the same terminal. The output of the controlleramplifier 50, like outputs of corresponding controller amplifiers (notshown) in controllers 24 and 26, are applied to the cathodes of diodes56, 58, which have a common anode connection. The diodes 56, 58, 60constitute output selector circuitry 28. The lowest of the signalsprovided at the output of the controller amplifiers forward biases oneof the three diodes while back biasing the other two. The lowest signalis passed through the forward biased diode on a lead 62 connecting theoutput selector circuitry 28 to a first switching means or mode control30, shown as a simple switch.

A second input to the mode control 30 is provided by the manual controlsubsystem 34 consisting of a voltage source 64 having a movable contact65 and a pair of positive and negative voltage sources. The movablecontact 65 is normally biased to a central, open-circuiting position butmay he held in an upper or lower position by an operator to complete anelectrical connection between contact 65 and the positive or thenegative voltage source, respectively. The contact 65 in the voltagesource 64 is connected to a switching means 66, hereafter referred to assecond switching means to distinguish it from the first switching meansin the mode control 30. When the system is operating in the automaticmode, the voltage source is open circuited and the movable contact inthe second switching means 66 is connected to COMMON. When the system isoperating in the manual mode, the current supplied by the voltage source64 is applied through the second switching means 66 to a first input 67to a manual control amplifier 68 having its output connected to the modecontrol 30. A second input 69 to amplifier 68 is connected to COMMON.The mode control 30 is connected to a current amplifier 70 whichsupplies current to the control element 32. A feedback resistor 72 isconnected in series with control element 32 between current amplifier 70and COMMON.

A system feedback path including a high input impedance amplifier 74connected to the upper end of feedback resistor 72 provides a feedbackcurrent to two branches. The first branch 75 includes negative feedbackpaths to each of the controllers 22, 24, 26. Referring to controller 22,the current is supplied through an adjustable resistor or potentiometer76 to a series-connected capacitor 78 in the controller feedback path.The controller feedback path may include a rate circuit 79 consisting ofa parallel combination of a capacitor 80 and a resistor 82 connectedacross a relay-controlled switch 84. The

feedback paths for controllers 24 and 26 are similar.

The system feedback current is also applied to a series-connectedcapacitor 86 in a second branch 88 of the system feedback path. Theupper end of the capacitor 86 is connected to the first input 67 to thesubsystem amplifier 68 and to the movable contact of the secondswitching means 66. An additional output limiting feedback circuit 51 isconnected across amplifier 50 in controller 22 to prevent the amplifier22 from saturating when controller 22 is idle. Similar limiting circuitsare connected across corresponding amplifiers in controllers 24 and 26.Since such limiting circuits are widely known in the art, theirconstruction and mode of operation are not described.

Automatic to Manual Transfer The circuit described above operates in thefollowing manner to provide substantially bumpless transfer from theautomatic mode to the manual mode of operation. While the system isstill in the automatic mode, current supplied to control element 32 isregulated by one of the controllers 22, 24, 26, each of which is biasedby a negative feedback current supplied by the first branch 75 of thesystem feedback path. A feedback current in the second branch 88 chargescapacitor 86 to a voltage V,. During the automatic mode, the upper plateof capacitor 86 and both inputs to amplifier 68 are connected to COMMON,making the voltage V, equal to the feedback voltage V, at the output ofthe high impedance amplifier 74 in the system feedback path.

At the time of transfer, the movable contacts in mode control 30 andswitching means 66 are moved to the M or manual position to connect theoutput of amplifier 68 to current amplifier 70 and the first input 67 ofamplifier 68 to the voltage source 64. Unless, however, the movablecontact 65 in source 64 is being held in contact with the terminal ofeither the positive voltage source or the negative voltage source, anopen circuit exists at movable contact 65. As a result, the voltage V isapplied directly to the first input 67 of amplifier 68. Before transfervoltage V, is the same as feedback voltage V and a very small errorvoltage is required at the input of amplifier 68 to apply a manual"signal to current amplifier 70 after transfer substantially equal to theautomatic signal previously applied by the active one of controllers 22,24, 26. A very small change in the output of amplifier 70 provides theerror voltage required at amplifier 68. Since the pre-transfer andpost-transfer outputs of current amplifier 70 are substantially equal,the transfer from the automatic mode to the manual mode is bumpless.

The preceding description indicates that the voltage applied to thefirst input of amplifier 70 following transfer is considered to besubstantially but not necessarily identically equal to voltage beforetransfer. Where the two voltages are unequal, due to the error voltagerequired at the input to amplifier 68, the pre-transfer andpost-transfer inputs to amplifier 70 will differ. The voltage at thefirst input 67 to amplifier 68 will be driven to the required level bymeans of normal feedback action. The change in output of power amplifier70 which is required to provide the error voltage at the input toamplifier 68 is negligible in comparison to the total output of poweramplifier 70 because of the high gain and low offset voltage ofamplifier 68. Therefore, the transfer is effectively, if not ideally,bumpless.

After the transfer is complete, an operator can control the currentapplied to amplifier 68 by holding movable contact 65 against theterminal for either of the voltage sources in source 64. The currentsupplied by the source 64 charges or discharges capacitor 86 in a rampmanner to the desired voltage.

Manual to Automatic Transfer While the system is operating in the manualmode, the rate circuit 79 in the feedback loop for controller 22 isshort circuited by switch 84. Both inputs to the controller amplifier 50and the left plate of the feedback capacitor 78 are connected to COMMONthrough switch 90. The feedback capacitor 78 charges to a voltagedetermined by the current through feedback resistor 72.

When the system is to return to an automatic mode of operation, themovable contacts in switches 30, 66, 84 and are thrown to the Aposition. The capacitor 86 is connected to COMMON while the input ofcurrent amplifier 70 is reconnected to the output of the selectorcircuitry 28. The opening of switches 84 and 90 causes the current fromthe feedback capacitor 78 to be summed with the deviation currentgenerated in differential error amplifier 40 at the junction 48. Sincethe input to current amplifier 70 from controller amplifier 50 followingtransfer to the automatic mode will differ from the pre-transfer inputsupplied by amplifier 68 by the amount proportional to the error voltageat the input of amplifier 50, the output of power amplifier 70 willchange slightly as the feedback voltage compensates for this errorvoltage. As was the case in the automatic-to-manual transfer, the changein output of power amplifier '70 is negligible relative to the totaloutput because of the high gain and low offset voltage of amplifier 50,making the manual to automatic transfer an effectively bumplesstransfer.

While there has been described what is believed to be a preferredembodiment of the present invention, variations and modifications willoccur to those skilled in the art once they become familiar with theinvention. For example, the invention would operate equally well insingle controller closed loop systems or in systems wherein controlmight be manually or automatically switched among multiple controllershaving dissimilar outputs. That is, the invention might be used wherethe output selector circuitry disclosed herein is replaced by a simplemanually controlled multi-position switch. Therefore, it is intendedthat the appended claims shall be construed to include all suchvariations and modifications as would occur to those skilled in the art.

lclaim:

1. For controlling the magnitude of current supplied to a controlelement, a system comprising:

a. a plurality of subsystems for alternatively controlling the current,each of said subsystems including an amplifier having a first inputterminal, a second input terminal which is electrically connected to asystem common terminal, and an output terminal, and

a feedback capacitor having one terminal connected to said first inputterminal; b. a system feedback path forming a feedback connectionbetween the control element and each of said feedback capacitors; c.system switching means for simultaneously connecting the output terminalof the amplifier in one of said subsystems to the control element,

connecting the first input terminal of each amplifier in the remainingsubsystems to the common terminal whereby the feedback current chargeseach of said feedback capacitors in each of the remaining subsystems toa voltage directly proportional to the current through the controlelement, and

disconnecting the first input terminal of the amplifier in said onesubsystem from the common terminal.

2. For controlling the magnitude of current supplied to a controlelement, a system including:

a. an automatic control subsystem comprising one or more processcontrollers, each of which includes an amplifier having a first inputterminal for receiving a signal representative of a deviation between apredetermined characteristic for the process and a setpoint for thatcharacteristic, a second input terminal connected to a system commonterminal, and an output terminal, and

a feedback capacitor having one terminal connected to said first inputterminal; b. a manual control subsystem including an amplifier having afirst input terminal, a second input terminal connected to the systemcommon terminal, and an output terminal, and

a feedback capacitor having one terminal connected to said first inputterminal;

c. a system feedback path connected between the control element and theother terminal of each of said feedback capacitors; and

d. system switching means for simultaneously connecting the outputterminal of an amplifier in a first of said subsystems to the controlelement, connecting the first input terminal of each amplifier in thesecond of said subsystems to the system common terminal whereby eachfeedback capacitor in said second subsystem is charged to a voltageproportional to the current through the control element, anddisconnecting the first input terminal of each amplifier in said firstsubsystem from the system common terminal. 3. A system as recited inclaim 2 wherein said manual control subsystem further includes a voltagesource comprising:

a positive voltage source; a negative voltage source; contact means forconnecting a selected one of said sources to the first input terminal ofthe amplifier in said manual control subsystem to control the currentsupplied to the first input, said contact means having a normalnon-connecting position from which it must be moved to effect theconnection.

4. A system as recited in claim 2 wherein said system feedback pathincludes a high impedance amplifier having an input terminal connectedin circuit with the control element and an output terminal connectedelectrically to each of said feedback capacitors in each of saidsubsystems.

5. A system as recited in claim 3 wherein said system feedback pathincludes a high impedance amplifier having an input terminal connectedin circuit with the control element and an output terminal connectedelectrically to each of said feedback capacitors in each of saidsubsystems.

6. A system as recited in claim 2 wherein said automatic controlsubsystem includes output selector circuitry responsive to apredetermined characteristic of amplifier output to pass the amplifieroutput having the most extreme value of that characteristic whileeffectively open circuiting amplifiers having outputs of less extremevalues.

7. A system as recited in claim 6 wherein said output selector circuitryincludes a plurality of diodes, each having a first terminal connectedto the output terminal of one of said amplifiers and a second terminalconnected in common with the second terminals of the other diodes to thesystem switching means.

t II i i

1. For controlling the magnitude of current supplied to a controlelement, a system comprising: a. a plurality of subsystems foralternatively controlling the current, each of said subsystems includingan amplifier having a first input terminal, a second input terminalwhich is electrically connected to a system common terminal, and anoutput terminal, and a feedback capacitor having one terminal connectedto said first input terminal; b. a system feedback path forming afeedback connection between the controL element and each of saidfeedback capacitors; c. system switching means for simultaneouslyconnecting the output terminal of the amplifier in one of saidsubsystems to the control element, connecting the first input terminalof each amplifier in the remaining subsystems to the common terminalwhereby the feedback current charges each of said feedback capacitors ineach of the remaining subsystems to a voltage directly proportional tothe current through the control element, and disconnecting the firstinput terminal of the amplifier in said one subsystem from the commonterminal.
 2. For controlling the magnitude of current supplied to acontrol element, a system including: a. an automatic control subsystemcomprising one or more process controllers, each of which includes anamplifier having a first input terminal for receiving a signalrepresentative of a deviation between a predetermined characteristic forthe process and a setpoint for that characteristic, a second inputterminal connected to a system common terminal, and an output terminal,and a feedback capacitor having one terminal connected to said firstinput terminal; b. a manual control subsystem including an amplifierhaving a first input terminal, a second input terminal connected to thesystem common terminal, and an output terminal, and a feedback capacitorhaving one terminal connected to said first input terminal; c. a systemfeedback path connected between the control element and the otherterminal of each of said feedback capacitors; and d. system switchingmeans for simultaneously connecting the output terminal of an amplifierin a first of said subsystems to the control element, connecting thefirst input terminal of each amplifier in the second of said subsystemsto the system common terminal whereby each feedback capacitor in saidsecond subsystem is charged to a voltage proportional to the currentthrough the control element, and disconnecting the first input terminalof each amplifier in said first subsystem from the system commonterminal.
 3. A system as recited in claim 2 wherein said manual controlsubsystem further includes a voltage source comprising: a positivevoltage source; a negative voltage source; contact means for connectinga selected one of said sources to the first input terminal of theamplifier in said manual control subsystem to control the currentsupplied to the first input, said contact means having a normalnon-connecting position from which it must be moved to effect theconnection.
 4. A system as recited in claim 2 wherein said systemfeedback path includes a high impedance amplifier having an inputterminal connected in circuit with the control element and an outputterminal connected electrically to each of said feedback capacitors ineach of said subsystems.
 5. A system as recited in claim 3 wherein saidsystem feedback path includes a high impedance amplifier having an inputterminal connected in circuit with the control element and an outputterminal connected electrically to each of said feedback capacitors ineach of said subsystems.
 6. A system as recited in claim 2 wherein saidautomatic control subsystem includes output selector circuitryresponsive to a predetermined characteristic of amplifier output to passthe amplifier output having the most extreme value of thatcharacteristic while effectively open circuiting amplifiers havingoutputs of less extreme values.
 7. A system as recited in claim 6wherein said output selector circuitry includes a plurality of diodes,each having a first terminal connected to the output terminal of one ofsaid amplifiers and a second terminal connected in common with thesecond terminals of the other diodes to the system switching means.